Sarsat Alt Cheap Bangla Desh

8/12/2019 Sarsat Alt Cheap Bangla Desh

1/6

978-1-4799-2385-4/14/$31.00 2014 IEEE

A Low Cost COSPAS-SARSAT Alternative forEPIRB Transponder for Local Fishing Boats in

BangladeshAbdul Quader Munshi1,*, Monalisha Mishu2and Kazi Abu Sayeed11North South University, Department of EECS, Dhaka, Bangladesh

2Daffodil Internationl University, Department of ETE, Dhaka, Bangladesh

*Corresponding author (E-mail : [email protected])

Abstract Every year pirates of Bay of Bengal attack a big

number of Bangladeshi fishermen at the sea and coastal areas.

Many fishermen already lost their lives and some were able to

save lives. With the advancement of technology, nowadays there

are various kind of Emergency Position Indicating Radio Beacon,

SAR Transponder, Emergency Personnel Locators available in

the market. One of the leading and most successful EPIRB is

COSPAS-SARSAT, a satellite based SAR Transponder which is

available for maritime use. But these devices are expensive and

not affordable by the fishermen living their life below poverty line

in Bangladesh. This emerges the need of a SAR Transponder,

which is relatively low in cost and also open format for platform

independent and readable by any radio operator on board of a

ship or shore. In this paper, we will discuss and propose of such

type of an alternate solution to the COSPAS-SARSAT

transponders, which is based on GPS and Radio Beacons,

relatively low in cost yet reliable.

KeywordsTransponder; SAR; EPIRB; GPS; RF Propagation

I. INTRODUCTIONBangladesh is a 3rd world country with a huge population

of fishermen who lives their life below poverty line. Every daythese fishermen goes to the sea for fishing and many times theyface threats by the local pirates of Bay of Bengal. Recent days,

piracy has been increased and many incidents have beennotified about fishermen in distress [1], [2]. Bangladeshscoastal area is around 103,800km

2 [3]. This emerges the need

of a distress signaling system for them, which will help thecoast guard to be notified, locate and rescue them.

Currently there are various Search and Rescue (SAR)Transponders and maritime distress signaling available.COSPASS-SARSAT [4] is one of the most popular and

effective for global coverage. Also GMDSS [5] is anotherpopular distress signaling mechanism for ship to shore distresssignaling. Both of them are expensive solution in terms ofBangladesh and this creates a need of an alternative EmergencyPosition-Indicating Radio Beacon (EPIRB) solution, which iseasy to build, operate and also effective at a budget, so that itcan be installed in every fishing boats.

In this paper, we will discuss the possible solutions for alow cost yet effective EPIRB for maritime use. In Section II,we will discuss in brief about the available solutions and theirworking principles, which guides the next steps of

development. Section III will give us a brief idea on thesystem block for the alternative EPIRB Transponder. In Section

IV will discuss on the message format that we will send to theshore as a distress code. Section V we will discuss several

proposals on RF and message encoding. Next, in Section VIwe will discuss about antenna systems for the transponder we

are proposing. In Section VII we will discuss about the shorestations playing role of mission control centre (MCC). InSection VIII we have discussed the current stage of the

prototype and Section IX describes and discusses the result ofthe first test run at field. Finally we will close our discussion byour conclusion on the topic, which will help us to design a lowcost maritime EPIRB, which may save a lot of life of thosefishermen in the sea.

II. CURRENTAVAILABLEMARITIMEDISTRESSSIGNALING

Currently there are two popular maritime distress signalingmethods are available and dominating with a great success. One

of them is COSPAS-SARSAT and the other one is theGMDSS. Both of them are discussed in brief below:

A. COSPAS-SARSATCOSPAS-SARSAT is a satellite based Search And Rescue

(SAR) program for distress information, which was establishedon 1979 by Canada, France, USA and former Soviet Union [4].Fig. 1 shows the most common type of products for this type ofdevice. These devices are very handy to use, battery operated.These normally does not require any switch to turn on and oncontact of sea water, they gets turn on and get the position ofthe beacon terminal using a GPS receiver embedded in it. Itthen sends the position data to the satellite on 406.0MHz with

an EIRP of 5W (a 100kHz band with centre frequency of406.05MHz). The transmitted distress message can be 112 bitsor 144 bits (short or long) of data [4]. Also some devicetransmits a homing beacon on 121.5 MHz, which is forsecondary basis as 121.5MHz is primarily used by civil aircraftdistress radio beacons.

Ref [4] has very detail of COSPAS-SARSAT with itsworking principle and system composition. Regardless of easeof use, each of these device price ranges from US$ 400.00 andthese requires registration to COSPAS in order to identify anyterminal activated in distress. Nowadays most of the merchant


2/6

marine vessels are equipped with it, but local fishermen inBangladesh are not equipped with it yet.

Figure 1. Common COSPAS-SARSAT devices

B. GMDSSThe Global Maritime Distress and Safety System (GMDSS)

is another form of distress signaling for marine vessels.GMDSS also works with the assistance of GPS. It uses DigitalSelective Calling (DSC) in marine HF and VHF channels.These are basically HF and VHF Radio Transceivers with thecapability of sending text and distress signals over narrow bandRadioTelex. These devices have a single button that requires to

be pressed during distress, so that any distress message alongwith the GPS coordinates will be instantly sent as a textmessage for direct printing to another GMDSS equipment. Ref[5] and [6] has very detail about this technology and productsavailable. Ref [6] and [7] tells that, a GMDSS station works onmarine VHF Ch 16 (156.80MHz) for voice and Ch 70(156.525MHz) for DSC signaling. Also for HF GMDSS radios,4125 kHz, 6215 kHz, 8291 kHz, 12.290 MHz and 16.420 MHzis used for long distance voice distress calls and 2187.5 kHz,4207.5 kHz, 6312 kHz, 8414.5 kHz, 12577 kHz and 16804.5kHz is used for DSC signaling.

GMDSS has total four areas of operation. Sea Area A1covers 30 nautical miles (56 km) to 40 nautical miles (74 km)from the shore station. VHF Ch 70 is used in this area fordistress operation with DSC. Sea Area A2 uses MF, 2187.5kHz in DSC and covers 180 nautical miles (330 km) of theoffshore during daytime and in nighttime it extends to up to 400nautical miles (740 km) due to good MF/HF propagation. SeaArea A2 excludes coverage of Sea Area A1. The other twoareas, Sea Area A3 and A4 are for more long distance withexclusion of A1, A2 for A3 and A1, A2, A3 for A4.Bangladeshi fishermen works mostly in Sea Area A1 and somego beyond A1, but does not reach A2.

Many of the merchant marine vessels nowadays areequipped with GMDSS radios, but still it is not so popular inBangladesh due to need of registration of each station. Also

GMDSS enabled radios are costly in comparison to Bangladesheconomy and especially for local fishermen.

III. SYSTEM BLOCKAs stated earlier, the goal is to send the position data of a

fishing boat in distress to shore over RF, the EPIRBTransponder will be installed on a fishing boat as a single box

device with built-in power source. The device will have onlyone switch, which will be used to turn ON the device to signal adistress. An additional switch will be there for a TEST beacon,which will also work in the same manner, but will not senddistress, instead it will exclusively send test beacon. The all-in-a-box device will only have at least 2 RF connectors, where onis for receiving GPS signals and the other one is to the beacon-transmitting antenna. In case of multiple transmissions like onefor distress position reporting and other for local homing

beacon (like COSPAS-SARSATs 406MHz and 121.5MHzpair), there may be multiple RF-OUT connectors for differentradiating antennas.

On pressing the distress button, a fully functional EPIRBwill be powered on and will lock itself with the GPS satellites.Once locked, the GPS data (position coordinates) will betransferred to the onboard processor and processor will convertthe position data into regular format from NMEA 0183 format[as most GPS gives readout in NMEA 0183 format]. The

processor will also prepare the distress message. Then theprocessor will encode the position report to the signallingmethod of desire like PSK or OOK, etc. After this, the encodeddata will be transmitted on the air using the radio transmitter(s)for sending the position data.

The proposed EPIRB will be comprised of the followingblocks:

A.

GPS Sub-systemThis sub-system is a plain GPS module capable of detecting

GPS satellites for Lat-Lon coordinates and local time. Theoutput is NMEA 0183 format, so that it is universal and easy tointerface with any system. While selecting a GPS module,

parameters like start-up time, satellite locking speed, powerconsumption, etc. should be carefully considered for price vs.

performance.

B. Controller Sub-systemThis is the heart of the whole system, which will be

connected to other sub-systems of the whole device. This isresponsible for entire message creation and encoding along

with sending data to transmitter and to act like a transponder bysending position data at an interval.

C. Radio Sub-systemThis part will be responsible for sending the encoded

message of distress prepared by the controller. In this case, thisis a pure transmitter, which will produce RF carrier and doother needful like modulation if needed. This part is basicallycomprised of sub-sub system of local oscillator and RF poweramplifier for minimum. In Section IV we will discuss moreon our proposals on RF sections and based on each proposal,


3/6

the sub-sub section will vary accordingly. The RF unit will beconnected to an antenna for radiating the signal.

The RF section can also be more than one unit (usually two)in case of availability local homing beacon.

Figure 2. System Simplified Block Diagram

D. Power Sub-systemThe power sub-section is self-explanatory. This will provide

power to the entire transponder to operate. This can be any DCpower source, inbuilt to make the entire transponder a singlebox device. This will be comprised of battery banks as powerstorage and also power supply circuits and power regulators forvarious sub-systems. For our particular case, we are not

proposing any PV array as an alternative power source for keepthe battery standby. As the device will be turned on duringdistress only, the power in the device will be stored for a longtime. But as batteries have self-discharge, we propose here forfirstly Li-Ion batteries for extended life and less weight. Alsowe are proposing to avoid hi grade Li-Ion batteries to reduce

price. Instead, a charging mechanism can be either inbuilt or a

separate charger can be used to charge the transponder duringboats maintenance time after each sail.

IV. MESSAGE FORMATA well-formatted message is required to be transmitted for

ease of use by almost any receiver. Here we are proposing amessage format, which will have A 4 digit ID for the MCC to

be called, which represents the parent region of operation. Thetransponder ID of 10 hexadecimal digits, where 3 are countrycode and rest 7 will bear transponders own ID. 9 characters fortimestamp in UTC, 6 characters for date, 8 characters forLatitude, 9 characters for Longitude, 3 characters of distresscode SOS.

In case of RTTY each character is made of 5 bits, with 1 bitfor start and 2 bits for stop mark for each character. Thus a bitstream of (8x4) + (8x10) + (8x9) + (8x6) + (8x8) + (8x9) +(8x3) = 390 bits are required to send the distress code.

V. PROPOSALSONRFSECTIONANDMESSAGEENCODING

Selection of a proper RF section is very essential along withselection of proper message encoding technique. As thetransponder will operate from a very long distance, and as it

will be relatively a very low power transmitter, there will benoise and interference naturally. Thus noise immunity is veryimportant for the success of the entire device operation.Different type of RF shows different type of properties atdifferent situations along with different mode of propagationand modulation. Depending on the best noise immunity, theerror rate of the message can be minimized. Our proposals aregiven below considering RF, modulation and message encoding

with the best practices:

A. CW Transmission over HF+VHFCW is the short form of Continuous Wave, which is nothing

but the emission of a plain carrier wave, unmodulated. Thismeans this is just the carrier frequency with no message in it atall [10], [11]. In general, when CW is used, it sends a character

by means of a series dot and dashes, which is the Morse code.As CW has plain carrier only, just keying the transmitter ONand OFF for proper timing can produce the desired message.When sending CW, also RF power (EIRP) will not become avery tight factor, as almost all of the available energy will beused to send the message.

Figure 3. RTTY message construction

HF is very popular with CW and has been used for manyyears by radio operators of marine vessels, land stations,amateur radio operators and military. In fact still CW is used inmany places for long distance communication among marine,military and amateur radio operators. For this transponder, weare proposing to utilize CW at a carrier frequency of 4125 kHzor 6215 kHz. Also other distress frequencies at HF range can beused. In this case we propose not to exceed frequency above 12MHz and to avoid the 2182 kHz, which is used for maritimemedium distance voice use. In this case the RF power should be5W at least with a good antenna for maximum RF emission.

Apart from the CW at HF, we also propose to have ahoming beacon at the VHF Ch 16. The homing beacon will

help to locate the transponder by any nearby marine vessel andshore stations including SAR teams on a chopper. The RFemission of this should be limited to 5W, and a 1W VHFtransmitter is more desirable to save power.

On both for the case of HF and VHF, we are selecting avoice distress channels instead of DSC channels because thiswill allow receiving the distress CW codes by anybody havingCW reception capability. This will also help to know otherlistening marine vessels on the same voice channel and to bealert and act accordingly. This will eventually help to


4/6

coordinate and recovering and locating the distressed boat byinter-disciplinarians.

Figure 4. NVIS propagation coverage

B. RTTY with HF + CW with VHFRTTY is the short form of Radio TeleType, a method of

sending texts as a form of digital code. AFSK or FSK is used tosend audio tone for mark and space to produce 1 and 0 andhence to send a character. Ref [12], [13] has a more details onRTTYs constructions and working principles. RTTY is alsovery popular in HF communication and is much more immuneto noise. The generation of RTTY signal is also relatively easy.As RTTY is an On-Off Keying (OOK), it requires nomodulation of audio tones (in this case two tones) with thecarrier. It is much better in terms of baud rate as it can have a

baud rate of 45.45, which is 60 words per minute in comparisonto CW [12], [13]. This makes it easier to work with any RTTYenabled HF receiver with a computer with software and a soundcard. Even as RTTY works on the main principle of CW, theOOK in practical with the overcoming of uneven coding of CWit is more better in terms of error free sending of characters. Asimple CW receiver or a BFO will also be able to assist inreception of RTTY if properly connected to a computerequipped with a sound card and proper software. In this case,the transmitter must be a CW generator, leaving the modulationaway. This will then become just another form of OOK, ratherthan long ON for dash and short ON for dot of a Morse code, itwill follow pure OOK. Thus will firstly reduce the complexityof the transmission part and will also assist in emission of the

maximum power, because OOK means just the plain carrier[13]. This makes the RTTY to be better choice over CW forboth noise immunity and less complex system.

The VHF transmitter will be used for a homing beacon asusual on marine VHF Ch 16. This will transmit a low speedCW to aid any nearby marine vessels to assist.

VI. ANTENNAANDRFPROPAGATIONFor a successful and efficient signal transmission to as much

as distance possible, a good matching between the transmitterand the antenna is a mandatory. Considering with the best

transmitters, a poor antenna or antenna system will lead tonowhere but very poor performance. A good antenna is alsoexpensive. Thus a trade-off will arise. But as our goal is to sendthe signal as much distance as possible, we can utilize severaltechniques altogether to get the maximum out of it.

The first parameter for an antenna is the size. As we havechosen a frequency at HF range for our long distance beaconsignal, we will require installing a larger antenna. In the case of4125 kHz, the wavelength is above 60 meters and below 80meters. This means the half wave antenna will be at least 30meters long and quarter wave will be 15 meter. In each case,the antenna length is totally impractical for wooden boats offishermen. Thus we first require shortening the antenna.Choosing a wave dipole will give us firstly a gain of 5.14dbi[17]. Ref [18] tells us that we can use series inductance toshortening the antenna length. Thus a loading coil will reducethe antenna length. Ref [18] also tells us that adding loadingcoil will reduce the antenna bandwidth, which will not affect usas we will be working on a single frequency. Thus with properloading coil, a reasonable size of antenna will make theinstallation much simpler. For the VHF range, another dipole of

wavelength can be used.

Figure 5. NVIS Propagation pattern with coverage range

For the most efficient RF emission, a proper impedancematching between the transmitter and the antenna is amandatory. Most of the case transmitters have an output

impedance of 50 . As dipoles are not exactly 50 , it requiresa matching unit known as BALUN to balance the feeder andalso to impedance match. Based on the antenna it will bechanged to match the most near SWR of 1:1 to avoid reflectionto the feed line. Antenna mounting and working with radiation

pattern is another key factor for achieving maximumtransmission range. For HF propagation, the vertical orientationof the antenna produces ground wave propagation, which has alast mile of around 50 km due to earth curvature. In horizontalorientation, the coverage range extends beyond 600 km. Thisintroduces a skip zone. Ref [19], [20] tells us about the NearVertical Incidence Skywave (NVIS) propagation in details,which helps us to eliminate the skip zone. In NVIS propagation,when the antenna is placed at a height of below wavelengthdown to above 1/20th wavelength. This creates the earthsurface as a reflector and sends most of the RF energy directlyupward just like a directional yagi-uda. The ion-layer acts asanother reflector, reflecting the signal back to earth surfacecovering a larger footprint. This method is well used bymilitary, UN and many others for vehicle and stationary HFapplications. Thus for our specific case, we are proposing to use

NVIS for the HF propagation. And for the VHF homing, avertical orientation is much more preferable as the polarizationand radiation both will support to get the maximum receivestrength for any nearby vessels.


5/6

VII. EPIRBMISSION CONTROL CENTREThe Mission Control Centre (MCC) will consist of basically

receiving systems as fixed installations. For any of the cases ofHF reception, a proper receiver will receive the distress signaland will decode the signal in the human readable form. For thiscase, a PC with software for CW/RTTY and a soundcard isenough. Apart from the HF installations, each MCC will alsohave receiver for 156.80 MHz local homing beacon signals. In

this case our proposal is to use a directional antenna with highgain and a beam width of maximum 180O. We also recommendestablishing several regional MCCs for better RF receptioncoverage. Also there can be ship-bourn MCCs by coast guard,those can be considered as Auxiliary MCC to aid regionalMCCs.

Figure 6. Test transmission at field, 4.5 kn from MCC

VIII. CURRENT STAGEAt current stage, instead of using any GPS, we first sent 100

sentences, ##S21VM: SAMPLE:1 OF SAMPLE:100;REMAINING SAMPLE:99## as the first attempt oftransponders data transmission. The ## is used for bracketingthe actual data so that it would be easy for us to recover even ina noisy or among other garbage data. These samples wereconsidered as position reports by the transponder. In this fieldtest, we first sent 100 samples (i.e. 100 position reports) to theMCC at an interval of 10 seconds. The payload size of the

position report was 54 ASCII characters. In RTTY format, eachcharacter consists of 5 bits and 3 additional bits, total 8 bits.Thus each payload had a total 432 bits. The transmission was at50 baud with following RTTY standards for mark and space.

For the first field test, the transponder hardware was amicrocontroller from Atmel Atmega 2650, a UHF walkie-talkiewith built-in antenna and transmission power of 4 watt. Wehave used AFSK for RTTY. Also the transponder was only ableto For the MCC, we have used a UHF walkie-talkie with built-in antenna at the height of a 9

th floor. For data processing and

extraction, we have used fldigi software in a laptop that iscapable of receiving RTTY in 50 baud. The transmission carrierwas at 433.00 MHz and the transponder was only based onRTTY only.

IX. RESULTSThe first field test run had both success and failure,

including the observation and proposals for tune-up,improvement, making more immune. During the first run, wehave sent the data from different distant locations, whichinclude 10.1 km, 4.5 km, 3.25 km, 2.25 km. Out of them, datafrom 3.25 km seemed with 98% success (lost sample# 1, 2) and2.25 km seemed 96% success (lost sample# 8, 15, 48, 51). We

have observed that the data gets only corrupted when wesimultaneously used a cell phone at a close proximity of theaudio cable that was used to feed the AFSK data to the laptop.Transmission from 4.5 km and above was a complete loss andwe have found that it was a line-of-sight issue as there was ahill between the transponder and the receiver, and the RF firingangle was not enough to skip over the hill as transmitter was allthe way kept at the side of the hill. Naturally in hilly or densetree areas, VHF is much better performer than UHF.

X. CONCLUSIONThe alternative EPIRB transponder of COSPAS-SARSAT

will serve as a locally developed transponder in low cost for thelocal fishermen. As this transponder will use the GPS for

positioning data, a boat in distress can easily send its position tomonitoring authorities on a single push of a button. Also formedium distance transmission, when HF with NVIS will beselected, the coverage range will be a broad range and ifmultiple MCCs are available or MCCs with Aux-MCCsavailable, all will get almost the same data, hence they candouble check for false alarm. Also since signal may be received

by more than one MCC, error in data can be easily detected, asin this case at least one MCC will have always better chance toreceive stronger signal than rest others. When each transponderwill have their own ID, maintaining a database and registration

process will make identification of the boat more user-friendly.

In a formatted distress message, for 3rd party listeners likemerchant vessels, amateur radio operators or other disciplines,it will be also easy to read the distress and relay the message toconcerning authority. For the data encoding, the best method

proposed here is RTTY for its simple construction, immunity tonoise. Also VHF homing will help as an auxiliary beacon toeven farther detection of a distress call.

The COSPAS-SARSAT transponders have a battery life ofaround 5 years and they require to inspection from time to timefor confirmed operation. As long lasting high power batteriesare expensive, the alternative approach can be recharging of

batteries during mooring of the fishing boats. This will reducethe price and will help to keep the power at hand.

Local fishing boats are made of mostly wood and are goodfor RF transmission as less chance of interference due to metal

body. Also as seawater is saline water, it can be a goodconductor and thus the need of a good ground becomes veryhandy by putting the ground wire in water. This will alsoimprove RF emission and overall, a relatively goodtransmission can be achieved with the position data, so thatmany lives can be saved who might be in distress at sea duringfishing.

According to the current study and field test, the resultcould be further improved if we had access to VHF and HF


6/6

transmitters. Also for transmission unit, a good dipole or loopwould give better performance than the built-in stubby

broadband antenna. And for MCC, a tuned dipole or loop withgood clearance would also improve the range further. Apartfrom that, the audio cable should be shielded and should have a

band-pass filter to filter out any extra noise (audio) so that theMCC computer will see a clean waterfall in the decodingsoftware.

APPENDIXI:LIST OF ABBREVIATIONS

Below are the abbreviations of the short forms used in thisdocument:

AFSK: Audio Frequency Shift Keying

BFO: Beat Frequency Oscillator

CW: Continuous Wave

DSC: Digital Selective Calling

EPIRB: Emergency Position-Indicating Radio Beacon

FSK: Frequency Shift Keying

GMDSS: Global Maritime Distress and Safety System

GPS: Global Positioning System

HF: High Frequency

MCC: Mission Control Centre

MF: Medium Frequency

NMEA: National Marine Electronics Association

NVIS: Near Vertical Incidence Sky wave

OOK: On-Off Keying

PSK: Phase Shift KeyingRTTY: Radio Tele-Type

SAR: Search and Rescue

SWR: Standing Wave Ratio

UHF: Ultra High Frequency

VHF: Very High Frequency

ACKNOWLEDGEMENT

We thank to Ocean Paradise Hotel & Resort, Coxs Bazar,Bangladesh, for their support through financing and providingfacilities in various ways to continue our research on this

project, that may help to save many lives in the coastal regions

of Bangladesh. Also we thank to Mr. Moududr Rahman fromthe ICT department, Ocean Paradise Hotel & Resort for hisclose support during the field test at the MCC part.

REFERENCES

[1] http://www.dw.de/pirates-rule-the-bay-of-bengal/a-16756484 [AccessedJan 20, 2014]

[2] http://www.natunbarta.com/english/opinion/2013/06/27/5742 [AccessedJan 20, 2014]

[3] UNEP/Earthprint, 2006, Assessing Coastal Vulnerability: Developing aGlobal Index for Measuring Risk, p. 30.

[4] International COSPAS-SARSAT Programme, (2014) The WikipediaWebsite. [Online]. Available: http://en.wikipedia.org/wiki/Cospas-Sarsat[Accessed Jan 20, 2014]

[5] Global Maritime Distress and Safety System, (2014) The WikipediaWebsite. [Online]. Available:http://en.wikipedia.org/wiki/Global_Maritime_Distress_and_Safety_System [Accessed Jan 20, 2014]

[6] ICOM Inc. Website. [Online]. Available at: http://www.icom.co.jp/world[Accessed Jan 20, 2014]

[7] International Distress Frequency, (2014) The Wikipedia Website.[Online]. Available:http://en.wikipedia.org/wiki/International_distress_frequency [Accessed

Jan 20, 2014][8] http://www.gpsinformation.org/dale/nmea.htm [Accessed Jan 20, 2014][9] NMEA 0183, (2014) The Wikipedia Website. [Online]. Available:

http://en.wikipedia.org/wiki/NMEA_0183 [Accessed Jan 20, 2014]

[10] Continuous Wave, (2014) The Wikipedia Website. [Online].Available: http://en.wikipedia.org/wiki/Continuous_wave [Accessed Jan20, 2014]

[11] http://www.w8ji.com/cw_bandwidth_described.htm [Accessed Jan 20,2014]

[12] Radioteletype, (2014) The Wikipedia Website. [Online]. Available:http://en.wikipedia.org/wiki/Radioteletype [Accessed Jan 20, 2014]

[13] (2014) RTTY Website. [Online]. Available:http://www.rtty.com/development/literature/introduction.htm [AccessedJan 20, 2014]

[14] (2014) ARRL Website. [Online]. Available: http://www.arrl.org/psk31-has-rtty-s-replacement-arrived [Accessed Jan 20, 2014]

[15] http://bpsk31.com [Accessed Jan 20, 2014][16] PSK31, (2014) The Wikipedia Website. [Online]. Available:

http://en.wikipedia.org/wiki/PSK31 [Accessed Jan 20, 2014]

[17] Antenna Gain, (2014) The Wikipedia Website. [Online]. Available:http://en.wikipedia.org/wiki/Dipole_antenna#Antenna_gain [AccessedJan 20, 2014]

[18] Electrical Length, (2014) The Wikipedia Website. [Online]. Available:http://en.wikipedia.org/wiki/Electrical_length [Accessed Jan 20, 2014]

[19] NVIS Theory, [Online]. Available:https://www.txarmymars.org/downloads/NVIS-Theory-and-Practice.pdf[Accessed Jan 20, 2014]

[20] Near Vertical Incidence Skywave, (2014) The Wikipedia Website.[Online]. Available:

http://en.wikipedia.org/wiki/Near_Vertical_Incidence_Skywave[Accessed Jan 20, 2014]

Sarsat Alt Cheap Bangla Desh

Documents

Transcript of Sarsat Alt Cheap Bangla Desh